A Russian transport helicopter has been shot down in opposition rebel territory in northern Syria, killing all five crew and officers onboard, the Kremlin said.
It is the deadliest single incident for the Russian military since its entrance into Syria's civil war.
The Mi-8 helicopter was shot down in Idlib province, where Russian warplanes frequently target rebels fighting Syrian president Bashar Assad's forces.
The Russian defence ministry said in a statement that the helicopter was returning to the Russian air base on Syria's coast after delivering humanitarian goods to the city of Aleppo. But video uploaded by opposition activists showed a rocket pod next to the wreckage
The helicopter had three crew members and two officers deployed with the Russian centre at the Hemeimeem air base on board, Russian officials said.
"From what we know from information provided by the defence ministry, all those who were on the helicopter died," Russian president Vladimir Putin's spokesman told journalists. The ministry statement released earlier said their fate was still unknown.
Mr Putin's spokesman, Dmitry Peskov, said the Russians "died heroically because they tried to move the aircraft away so as to minimise losses on the ground".
There was no immediate claim of responsibility for the attack.
Idlib province has a strong presence of fighters both for the al Qaida branch in Syria known as the Nusra Front and other groups fighting against Assad's forces. The Nusra Front announced last week that it was changing its name and relinquishing ties with al Qaida in an attempt to undermine a potential US and Russian air campaign against its fighters.
The group is part of a coalition of insurgent groups called Jaish al-Fateh, or Army of Conquest, which has captured most of Idlib.
International human rights groups have repeatedly accused Russia of using cluster munitions against civilians and rebels since it began its aerial campaign in Syria last year to shore up Assad's forces. Russian officials say they are fighting extremists in Syria, but have frequently targeted more moderate elements of the Syrian opposition.
Videos uploaded online by Syrian opposition activists show the burning wreckage of a Russian helicopter in footage seemingly taken in the first few moments after the helicopter crashed.
In one video, a rocket pod can be seen next to the wreckage. People standing nearby are seen taking mobile phone photos and shouting Allahu Akbar, or God is great in Arabic.
The helicopter appears to have broken up as it crashed, its tail can be seen lying separately from the aircraft's body in flames. In other videos, the body of one purported Russian soldier is seen being dragged by the legs while an unidentified person stands on the body of another soldier, also purportedly Russian.
Monday's helicopter downing was the deadliest for the Russians since Moscow began carrying out air strikes in Syria in support of Assad's forces last September.
In July, two Russian airmen were killed in the central Homs province when their Mi-25 helicopter was shot down by what the defence ministry said were IS fighters.
An Mi-28N helicopter gunship crashed near Homs in April, killing both crew members, but the Russian military said there was no evidence it came under fire.
A Russian warplane was shot down by Turkey along the Syrian border in November, and one of the two pilots was shot and killed from the ground after ejecting.
Earlier on Monday, a Syrian military official said that government forces repelled an attack by insurgents that was an attempt to break the siege imposed on rebel-held parts of the northern city of Aleppo.
The development came a day after Syrian rebels launched the offensive to break up the government's siege of the eastern, rebel-held part of the city.
The UN estimates some 300,000 people are still trapped in the rebel section of Aleppo, with dwindling food and medical supplies. The UN's special envoy to Syria, Staffan de Mistura, warned on Friday that basic supplies in eastern Aleppo could run out in three weeks.
Opposition activists said intense fighting was still ongoing in Aleppo on Monday.